During the past several years, microprocessors have been used more and more for controlling hydraulic valves for automating many hydraulic circuits. The microprocessor analyzes the data received from various pressure and/or flow sensors strategically placed within the circuit and directs a command signal to actuate the proper control valve. One of the problems encountered is that the microprocessor is normally digital and deals in discrete signals while the control valves heretofore have been analog in nature. Such valves are not readily compatible with digital electrical control signals and are prone to be sensitive to contamination.
In some electro-hydraulic systems the interface between the electronics and the hydraulics is a simple on/off solenoid valve in which a solenoid is connected to a spool of a hydraulic valve. Energizing the solenoid moves the valve spool to an on or open position in which fluid flow therethrough is established. Conversely, deenergizing the solenoid moves the valve to the off or closed position at which fluid therethrough is blocked. A problem encountered with such solenoid valves is that the response time (response time is considered the time lapse between the instant the electrical signal either reaches or is cut off from the solenoid and the instant the valve reaches either the on or off position) thereof is not fast enough for fine control of some actuators. For example, the minimum time that the valve spool is in the open position (pulse width) is dependent upon the time required for the magnetism in the coil of the solenoid to decay once the electrical signal is cut off. Thus, even if the electrical signal to the solenoid is cut off at precisely the same instant the valve reaches the open position, the solenoid valves commonly require a time greater than 30 milliseconds before the valve starts to move toward the closed position. While special high speed solenoids are available, the cost thereof would be prohibitive for most applications.